It is often desirable for a laser to emit coherent electromagnetic radiation at either of two different, widely-separated wavelengths. In many laser applications laser radiation at one wavelength would be desirable for performing one function, and laser radiation at another, much shorter wavelength would be desirable for performing another function. As one example, in machining metals with lasers it can happen that solid metal melts readily when exposed to laser radiation at one wavelength but that molten metal remains in a molten state and vaporizes more readily when exposed to laser radiation at another wavelength much longer than the first wavelength. As another example, in laser etching of layers of a photoresist it can happen that a layer of photoresist vaporizes when exposed to laser radiation at a first wavelength and that the gaseous vapors absorb laser radiation at the first wavelength while transmitting laser radiation at a second wavelength much shorter than the first wavelength. In such an application it is convenient to use laser radiation at the first wavelength to vaporize the photoresist and then to use laser radiation at the second wavelength to perform operations on materials from which the layer of photoresist has been vaporized.
In many of these applications rapid switching of the laser output between the two wavelengths would be quite useful. Unfortunately, presently available laser systems do not provide rapid switching between two widely-separated wavelengths.
Many lasers change from one output wavelength to another output wavelength only by mechanical motion of some part. For many applications, including precision applications, mechanical movements connected with tuning between the two wavelengths are detrimental. The speed of mechanical movements is limited by the inertia of parts which must be moved. Mechanical movements can also create undesirable vibrations.
In other types of lasers the wavelength at which lasing action occurs can be changed by changing the gain medium. For example, the wavelength at which lasing action occurs can be changed in a dye laser by changing the dye and in a gas laser by changing the gas. Such ways of changing the wavelength are usually relatively slow.
In addition, solid-state optical parametric oscillation lasers have a very wide tuning range, but the power levels and stability of such lasers are unsuitable for many industrial applications.
Some workers have developed lasers which lase at two or more wavelengths simultaneously. In such lasers the range of wavelengths at which simultaneous lasing occurs is often relatively narrow. Moreover, in such lasers it may be difficult to switch quickly between these laser wavelengths.
Other workers have developed lasers in which the lasing action can be tuned over a range of wavelengths. Such lasers include titanium-sapphire tunable lasers and chromium-alexandrate tunable lasers. The tuning entails mechanical motion, and the tuning range is usually relatively narrow.
For other tunable laser systems, important parameters such as power and pulse width are not acceptable for some industrial applications.
There is accordingly a need for a laser having an output which is rapidly switchable between a first wavelength and a second, much shorter wavelength without mechanical movements.
Those working in applications of lasers have long known of Q-switching and of its practical uses. Q-switching produces a high-intensity pulse of coherent electromagnetic radiation from a laser. The laser cavity is initially maintained in a condition in which lasing is inhibited while the gain medium is pumped with input energy. In that condition a substantial population inversion accumulation is created and maintained in the gain medium. The state of the laser cavity is then changed to a condition in which lasing is no longer inhibited. The gain medium then quickly releases the energy stored in the population inversion accumulation in a high-intensity pulse of coherent electromagnetic radiation. The peak power delivered from a laser by such a Q-switched output is considerably higher than the power delivered in continuous-wave operation of the laser.
Because Q-switching has a wide variety of important practical applications, a laser which is rapidly switchable between a first wavelength and a second, much shorter wavelength should also have the capability of delivering Q-switched output pulses at each of those two wavelengths.
In many practical laser applications, and particularly in laser machining, it is necessary to achieve a power level in a beam of coherent electromagnetic radiation which is high enough to perform a desired operation on a target. There are two ways to produce such a beam.
First, a gain medium may be selected which produces a beam at a desired wavelength.
Second, frequency conversion (e.g., doubling, tripling, quadrupling, or two wave mixing) may be used to generate a beam at a desired wavelength from a beam at a different (usually longer) wavelength. As an example, the gain medium neodymium yttrium-aluminum-garnet ("Nd:YAG") produces a high-power output beam at a wavelength of approximately 1064 nanometers. That beam is then passed through a frequency doubler, which generates from the beam at 1064 nanometers a coherent output beam at approximately 532 nanometers. The output beam at 532 nanometers is then used to perform the desired operations, such as laser machining of a workpiece.
A first prior art method of controlling laser beam power is to use a conventional polarization-based attenuator that includes a polarization state changer located between a polarizer and a crossed analyzer. The polarization state changer rotates the polarization direction of the beams passing through the polarizer so that the analyzer produces a desired degree of attenuation of the beams incident to it. The polarization state changer must withstand high-power beams. This limits the usefulness of conventional polarization state changers such as liquid crystals, which can be used only at very low power levels because of their low damage thresholds. This prior art method, if used in an intracavity location, can also impair the stable operation of the gain medium.
A second prior art method of controlling the power is to vary the pumping energy which drives the gain medium. This method is limited by the narrow range of stable operation of many laser devices.
A third prior art method of controlling the power is to use movable laser mirrors with a surface having different attenuation ratios at different locations. The mirror is moved so that the beam strikes the mirror at a location with the desired attenuation ratio. The required mechanical motions make this third prior art method inherently slow. In addition, where several mirrors are used to achieve a variety of different attenuation combinations, it is difficult to maintain the necessary accuracy in the location of the beam path.
A fourth prior art method of controlling the power is to use an acousto-optical attenuation device. However, to realize a full dynamic range of power control (that is, to control the power from zero to 100 percent of maximum output power), only the first order of the beam deflected by the acousto-optical attenuation device can be used as the working beam. The maximum output power in the first order beam deflected by an acousto-optical attenuation device is only a few percent of the total input beam power available. Thus, power control using an acousto-optical attenuation device is feasible only for applications which require a low power level.
There is a variety of practical problems encountered in laser machining which cannot be overcome because of the foregoing limitations.
When Q-switched pulses are used in industrial applications such as laser machining, the area illuminated by the laser beam is moved slowly over a surface, or through a region, of a workpiece, with the Q-switched pulses forming a series of overlapping spots. To achieve uniform and predictable machining of a workpiece when such a series of overlapping spots is used, it is important that the power delivered by the beam be uniform among the pulses at different times. A particular problem in achieving such uniformity is caused by differing time intervals between pulses. The power released by a gain medium in a Q-switched output pulse depends on the time the gain medium has been pumped before that Q-switched output pulse.
The time between Q-switched output pulses of a laser used for laser machining varies considerably during the course of many machining operations. First, the first Q-switched pulse of a machining operation usually has considerably more power than succeeding pulses because the gain medium has been pumped for a relatively long time before the first Q-switched pulse. To eliminate this high-power pulse, many laser machining systems simply aim the laser beam away from the workpiece for that first pulse. This initial step is inconvenient because it requires additional operations and equipment. Second, during the process of machining a workpiece, the Q-switched pulse repetition rate typically varies as the laser beam carries out different types of machining operations. For example, as the laser beam turns a corner over the surface of a workpiece, the repetition rate typically decreases, often because a longer time is required to move the area or region to be illuminated by the beam around a corner than that required to move in a straight line.
There accordingly is a need for apparatus capable of providing precise control of the power in Q-switched pulses of coherent beams. There is a similar need when the beam is produced in continuous-wave operation. In addition, any such apparatus should be capable of adjusting the power in Q-switched pulses on a time scale faster than the pulse repetition rates encountered in typical laser machining applications. Pulse repetition rates of several kilohertz are common in laser machining applications, and any apparatus to control the power in a coherent beam should be able to vary the power level between adjacent pulses of a pulse train at such a repetition rate.